Avoid DCHECK failure for chrome:// URLs in autocomplete suggestions

components/omnibox/browser/autocomplete_result.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/omnibox/browser/autocomplete_result.h"
 
 #include <algorithm>
 #include <iterator>
 
 #include "base/command_line.h"
@@ skipping 148 matching lines @@
   if (default_match_ != matches_.end()) {
     const base::string16 debug_info =
         base::ASCIIToUTF16("fill_into_edit=") +
         default_match_->fill_into_edit +
         base::ASCIIToUTF16(", provider=") +
         ((default_match_->provider != NULL)
             ? base::ASCIIToUTF16(default_match_->provider->GetName())
             : base::string16()) +
         base::ASCIIToUTF16(", input=") +
         input.text();
-    DCHECK(default_match_->allowed_to_be_default_match) << debug_info;
-    // If the default match is valid (i.e., not a prompt/placeholder), make
-    // sure the type of destination is what the user would expect given the
-    // input.
-    if (default_match_->destination_url.is_valid()) {
-      if (AutocompleteMatch::IsSearchType(default_match_->type)) {
-        // We shouldn't get query matches for URL inputs.
-        DCHECK_NE(metrics::OmniboxInputType::URL, input.type()) << debug_info;
-      } else {
-        // If the user explicitly typed a scheme, the default match should
-        // have the same scheme.
-        if ((input.type() == metrics::OmniboxInputType::URL) &&
-            input.parts().scheme.is_nonempty()) {
-          const std::string& in_scheme = base::UTF16ToUTF8(input.scheme());
-          const std::string& dest_scheme =
-              default_match_->destination_url.scheme();
-          DCHECK(url_formatter::IsEquivalentScheme(in_scheme, dest_scheme))
-              << debug_info;
+
+    // Ensure there is a default match if (and only if) the omnibox is not
+    // empty. Typically, the omnibox will never be empty because it is filled
+    // with the current page URL when the omnibox is focused. The zero-suggest
+    // default match, if any, should navigate to this URL. However, on some
+    // WebUI pages (eg: chrome://newtab) the URL is not auto-filled. In this
+    // case, there should be no default match.
+    if (input.text().empty()) {

[Peter Kasting, 2016/09/01 22:06:35]

Nit: Less nesting and code duplication.  (I rewrote the comments a bit as well,
especially to provide more detail on when we get into different scenarios and
why we do or don't want default matches.)

    // We should only get here with an empty omnibox for automatic suggestions
    // on focus on the NTP; in these cases hitting enter should do nothing, so
    // there should be no default match.  Otherwise, we're doing automatic
    // suggestions for the currently visible URL (and hitting enter should
    // reload it), or the user is typing; in either of these cases, there should
    // be a default match.
    DCHECK_NE(input.text().empty(), default_match_->allowed_to_be_default_match)
        << debug_info;

    // For navigable default matches, make sure the destination type is what the
    // user would expect given the input.
    if (default_match_->allowed_to_be_default_match &&
        default_match_->destination_url.is_valid()) {
      ...

[mattreynolds, 2016/09/02 00:29:08]

Done.

+      DCHECK(!default_match_->allowed_to_be_default_match) << debug_info;
+    } else {
+      DCHECK(default_match_->allowed_to_be_default_match) << debug_info;
+      // If the default match is valid (i.e., not a prompt/placeholder), make
+      // sure the type of destination is what the user would expect given the
+      // input.
+      if (default_match_->destination_url.is_valid()) {
+        if (AutocompleteMatch::IsSearchType(default_match_->type)) {
+          // We shouldn't get query matches for URL inputs.
+          DCHECK_NE(metrics::OmniboxInputType::URL, input.type()) << debug_info;
+        } else {
+          // If the user explicitly typed a scheme, the default match should
+          // have the same scheme.
+          if ((input.type() == metrics::OmniboxInputType::URL) &&
+              input.parts().scheme.is_nonempty()) {
+            const std::string& in_scheme = base::UTF16ToUTF8(input.scheme());
+            const std::string& dest_scheme =
+                default_match_->destination_url.scheme();
+            DCHECK(url_formatter::IsEquivalentScheme(in_scheme, dest_scheme))
+                << debug_info;
+          }
         }
       }
     }
   }
 
   // Set the alternate nav URL.
   alternate_nav_url_ = (default_match_ == matches_.end()) ?
       GURL() : ComputeAlternateNavUrl(input, *default_match_);
 }
 
@@ skipping 182 matching lines @@
        i != old_matches.rend() && delta > 0; ++i) {
     if (!HasMatchByDestination(*i, new_matches)) {
       AutocompleteMatch match = *i;
       match.relevance = std::min(max_relevance, match.relevance);
       match.from_previous = true;
       matches_.push_back(match);
       delta--;
     }
   }
 }